Means for prevention of leakage of high frequency electromagnetic waves will be described herein by taking, as an example, a microwave oven, which cooks food by dielectrically heating it by high frequency electromagnetic waves.
A microwave oven comprises a heating chamber for receiving food for heating it by high frequency waves, and a door adapted to open and close the opening provided in said heating chamber for putting in and taking out food to be cooked, wherein an electric wave sealing measure is taken to prevent high frequency electromagnetic waves in the heating chamber from leaking outside the chamber to hurt the human body when the door is opened to put in and take out food.
As another example, in waveguides utilized in communications systems, an electromagnetic sealing device is provided in the mechanical junction between the waveguides.
As an example of prior art, U.S. Pat. No. 3,182,164 is shown in FIG. 1. In FIG. 1, the numeral 1 denotes a heating chamber for a microwave oven, and a door 4 having a knob 3 is provided for covering the opening 2 in said heating chamber 1 so that the opening can be opened and closed. The peripheral edge of the door 4 is formed with a hollow choke portion having a clearance 5 which opens toward the heating chamber 1. The depth 7 of the choke portion is designed to be substantially 1/4 of the wavelength of the high frequency wave to be used. In this case, the thickness of the door 4 is also 1/4 of the wavelength. That is, since the frequency of the electromagnetic wave heretofore used in microwave ovens is 2450 MHz, 1/4 of the wavelength is about 30 mm. In order to be opposed to the choke portion 6 of this length, the peripheral edge 8 formed around the opening 2 in the heating chamber 1 has a thickness 9 which is greater than 1/4 of the wavelength. Therefore, the effective size of the opening 2 in the heating chamber 1 is smaller by an amount corresponding to the size of the peripheral edge 8.
As another example of prior art, U.S. Pat. No. 2,500,676 is shown in FIGS. 2(a) and (b). This example also shows a construction for microwave ovens, wherein high frequency waves resulting from the oscillation of a magnetron 10 are fed to a heating chamber 11 to cook food 12. The opening 13 in this heating chamber 11 is provided with a door 14 covering the opening 13 so that the latter can be opened and closed. The peripheral portion of this door 14 is formed with a choke portion 15 in the form of a groove to prevent leakage of high frequency waves. The depth of this choke portion 15 is also designed to be 1/4 of the wavelength of the high frequency wave to be used. As a result, the effective size of the opening 13 is correspondingly smaller than that of the heating chamber 11.
As described above, the conventional choke portions are based on the technical concept that 1/4 of the wavelength is essential for attenuating the high frequency waves.
Thus, let Z.sub.0 be the characteristic impedance of the choke portion, and L be its depth. Then, when the terminal end portion is shorted, the impedance Z.sub.IN at the choke opening is given by ##EQU1## (where .lambda..sub.0 is the free space wavelength.)
The electric wave attenuating means of the choke type is based on the principle of selecting the depth L of the choke so that it is 1/4 of the wavelength, thereby achieving. ##EQU2##
If the choke is filled with a dielectric (the specific dielectric constant of the material: .epsilon..sub.r), the wavelength .lambda.' of the electric waves is reduced to ##EQU3## In this case the depth L' of the choke portion is reduced as shown below. ##EQU4## However, the relation L'=.lambda.'/4 is still retained, so that in the choke system it is impossible to make the depth substantially less than 1/4 of the wavelength, imposing limitations on the reduction of the size of the choke portion.
The choke system is based on the known 1/4 wavelength impedance conversion principle. As shown in FIG. 3, let Z.sub.0C be the characteristic impedance, l.sub.C be the depth of the groove, Z.sub.0P be the characteristic impedance of a leakage path 17 extending from the heating chamber to the choke groove, l.sub.p be the length of said leakage path 17, and .lambda. be the wavelength to be used. Then, as shown in FIG. 3, the shorted impedance (Z.sub.C =0) of the bottom C of a choke groove 18 is ##EQU5## at the opening B in the choke groove 18. The numeral 19 denotes the heating chamber of the microwave oven, and 20 denotes a door. By selecting ##EQU6## the conversion .vertline.Z.sub.B .vertline.=.infin. is possible. The impedance Z.sub.A when the impedance Z.sub.B of the opening B is looked at from the line starting point A is ##EQU7##
By selecting ##EQU8## the conversion .vertline.Z.sub.A .vertline.=0 is possible. Thus, the ingenious utilization of the 1/4 wavelength impedance inversion principle enables the shorted state at the bottom of the choke groove 18 to appear at the line starting point, whereby the electromagnetic wave energy seal arrangement can be put to practical use.
By filling the leakage path 17 and choke groove 17 with a dielectric having a specific dielectric constant .epsilon..sub.r, the wavelength .lambda.' is made .lambda..sqroot..epsilon..sub.r, but the same effect can be obtained by using the 1/4 wavelength (.lambda.'/4) impedance principle.
In addition, as another example of prior art, there is U.S. Pat. No. 3,584,177.
Further, there is an example of prior art such as U.S. Pat. No. 3,511,959 wherein impedance inversion is effected by changing the characteristic impedance of the leakage path at every unit length of .lambda./4.
As for a method of impedance inversion of transmission path length using .lambda./4 as a unit wherein the leakage transmission path is made a parallel transmission system having a narrower width than the aforesaid transmission path and the leakage propagation mode is a TEM wave, there are U.S. Pat. Nos. 2,772,402 and 2,850,706 and U.K. Pat. No. 1,022,103.
In the case of the references cited above, it is necessary that the length of the transmission path be at least .lambda./4.
The electromagnetic wave energy seal arrangements based on the length .lambda./4 have superior performance and are capable of maintaining the design performance for long periods of time, so that they have often been employed in microwave communications systems and high frequency heating apparatuses. However, as is clear from the prior art item described above, there has been a disadvantage the length cannot be made less than .lambda./4.